Description of Related Art
Read disturb presents a serious problem to the operation of nonvolatile memory cells such as floating gate and charge trapping memory cells. Read disturb occurs when read operations are performed on nonvolatile memory cells; despite being applied for a read operation rather than a program operation, some programming still occurs while applying a read bias arrangement. Over the course of many read operations, read disturb changes raise the threshold voltage of the affected nonvolatile memory cells.
Read disturb occurs in NAND strings because the read bias arrangement voltage Vpass is sufficiently high to cause programming. In a NAND string of nonvolatile memory cells arranged in series, a read voltage Vread is applied to the word line of a selected memory cell in the NAND string, and a pass voltage Vpass is applied to the word line of unselected memory cell in the NAND string.
FIG. 1 is a graph of threshold voltages distributions, showing the HVt and LVt distributions, the word line read voltage window and the word line pass voltage window. Vpass is sufficiently high to turn on the channel under unselected memory cells in the NAND string regardless of the data values stored on the unselected memory cells. In particular, Vpass is sufficiently high to turn on the channel under a memory cell storing the data value associated with the highest threshold voltage distribution. Vread is sufficiently high to turn on the channel under a selected memory cells in the NAND string with a data value associated with a threshold voltage distribution that is lower than Vread, and is sufficiently low to turn off the channel under a selected memory cells in the NAND string with a data value associated with a threshold voltage distribution that is higher than Vread.